The present invention relates to a navigation system, and more particularly to a navigation system incorporating a global positioning system (GPS) receiver and a self-operating sensor for determining the position of a reception point or a mobile object such as an automobile in a global geometrical region.
There are known navigation systems incorporating global positioning system (GPS) for navigating various mobile objects such as automobiles, airplanes, ships, or the like in a global geometrical region. Typically, such a navigation system on a mobile object has a GPS receiver which receives radio waves transmitted from three or more GPS satellites, determines the position of a reception point where its radio-wave receiver has received the radio waves, based on quasi-distance data between the GPS satellites and the reception point, including a time offset of the receiver, and positional data of the GPS satellites, and outputs the data about the determined position. Since, however, the radio waves from the GPS satellites may not necessarily be received under good conditions because of receiving environments and weather conditions, some navigation systems which incorporate a GPS receiver are also combined with a self-operating sensor which produces the positional data of its own. In operation, the navigation system selects whichever of the data from the GPS receiver and the self-operating sensor is more accurate at the time for higher.positioning accuracy for better navigation. The self-operating sensor for use in automobile navigation systems may comprise an orientation sensor such as a geomagnetic sensor or a gas-rate gyroscope, or a speed sensor such as a sensor for detecting the rotational speed of a crankshaft.
One known navigation system comprising a GPS receiver and a self-operating sensor is disclosed in Japanese laid-open patent publication No. 60-239794, for example. The disclosed navigation system employs a PDOP (Position Dilution Of Precision) value for determining the accuracy with which the GPS receiver determines the position. If the PDOP value is equal to or lower than a predetermined value, then the navigation system selects the data from the GPS receiver for navigation. The PDOP value is used in a three-dimensional positioning system in which the three-dimensional position of a reception point is determined by simultaneously measuring the distances up to four or more GPS satellites. The PDOP value is representative of how positional errors of the GPS satellites are reflected by the calculated present position of the reception point. If the PDOP value is larger, then it indicates that the calculated present position of the reception point is suffering a greater error.
Inasmuch as the known navigation system determines the positional accuracy of the data according to the PDOP value, however, when only a two-dimensional positioning process is available for some reason, i.e., when the PDOP value cannot be obtained, then the navigation system cannot detect a reduction in the positional accuracy owing to an error caused by a change in the altitude of the reception point. Another problem is that in the event of an intentional accuracy reduction known as selective availability (SA), the PDOP value itself is caused to contain an error, resulting in a lower degree of positional accuracy. The navigation system determines whether it is to select the data from the GPS receiver or the data from the self-operating sensor, solely based on the positional accuracy of the GPS receiver. Consequently, once the positional accuracy of the data from the GPS receiver is determined to be low according to the PDOP value, the navigation system automatically selects the data from the self-operating sensor even when the positional accuracy of the data from the self-operating sensor is actually lower than the positional accuracy of the data from the GPS receiver.